Three-phase power rectifiers with which a direct-current can be transformed into an alternate-current voltage and vice versa have been known. Considering a two-phase embodiment of the power rectifier, three branches are connected parallel to each other with the use of respectively two power semiconductor elements. For example, the power semiconductor elements may be so-called IGBTs (IGBT=insulated gate bipolar transistor), each of which can be connected in parallel to an oppositely poled diode. An intermediate circuit may be connected in parallel to the three branches, i.e., in particular in the form of a capacitor. The three connecting points of the respectively two serially connected power semiconductor elements of the three branches can be connected to the phases of a three-phase electric motor or generator, for example.
If one of the two power semiconductor elements and/or the associate diode in one of the branches of the power rectifier is defective, a branch short circuit occurs in a downstream limit switch arrangement of the other power semiconductor element of the branch. This means that the parallel-connected capacitor is short-circuited via the defective element and the conductively switched other power semiconductor element of the branch is short-circuited.
Methods wherein such a branch short circuit can be detected have been known. For example, regarding this, it has been known to monitor the gate-emitter voltage or the collector current when IGBTs are used. If such a branch short circuit is detected, it has also been known to again switch at least the other—conductively connected—power semiconductor element of the branch non-conductive, namely to switch it off. In addition, it has been known to also switch off all the other power semiconductor elements or to bar them from being switched on. Therefore, it is possible to avoid a destruction of additional power semiconductor elements.
However, via the defective power semiconductor element and/or its associate diode of the one branch of the power rectifier, a short-circuit current continues to flow, i.e. via the connected electric motor or the generator and via the diodes in the two other branches. In doing so, this short-circuit current via the electric motor or the generator may follow an asymmetric course, wherein specifically the alternative-current component may be superimposed by a direct current. This may lead to critical torque changes in the electric motor or the generator and specifically result in oscillating torques.